This invention relates to a microfabricated transducer used for sensing the direction and magnitude of a magnetic field.
Microelectromechanical systems are devices which are manufactured using lithographic fabrication processes originally developed for producing semiconductor electronic devices. Because the manufacturing processes are lithographic, MEMS devices may be made in very small sizes. MEMS techniques have been used to manufacture a wide variety of transducers and actuators, such as accelerometers and electrostatic cantilevers.
Magnetically sensitive structures employing effects such as anisotropic magneto resistance (AMR) and giant magnetoresistance (GMR) are well known in the field of magnetic recording, and may be fabricated using MEMS and integrated circuit lithographic processes. These microfabricated devices provide a response to an applied magnetic field, for example, that arising from individual domains created in a magnetic recording medium. Because of their small size, the AMR and GMR sensors can resolve very small bit sizes and thus increase the density and storage capacity of the recording medium. Accordingly, these devices are suitable for detecting relatively large changes in flux density over a relatively small spatial dimension. However, when these transducers are used to detect slowly varying, extensive magnetic fields, such as the earth's magnetic field, their small size limits their performance. Furthermore, extensive fields may give rise to noise in the sensor, such that measures are often taken to shield the sensor from such fields.
For example, U.S. Pat. No. 7,898,249 to Paul, et al. discloses a microfabricated magnetoresistive (AMR) and giant magnetoresistive (GMR) transducer which uses a soft magnetic material to shield the transducer from stray or long ranging magnetic fields. U.S. Pat. No. 7,898,249 is incorporated by reference in its entirety. Because of the presence of the shields, magnetic fields between the closely spaced shields are substantially perpendicular to the direction of the gap between the respective flux guides, and thus arise primarily from the domains created in the data storage medium.
Hall effect sensors can also be used to detect the position of a small magnet on a movable component, as described for example, in U.S. Pat. No. 8,285,328. Because of the low output, these sensors generally must be disposed in close proximity to the magnet, and are thus also not useful for detecting or measuring extensive magnetic fields.
Because the size of these microfabricated devices is small, when making field sensing devices, the capture area of the device within that field may also be small, and therefore the signal or output of the sensor may be small as well.
Furthermore, prior art magnetic field transducers which are capable of detecting and measuring components of a magnetic field lying along any or all of the three orthogonal spatial axes, x-, y- and z- have required the fabrication and packaging of three separate, independent transducers. Current technology either packages several transducer dies each sensitive in the x-, y- and/or z-directions into one package, thus increasing packaging cost and complexity, or are utilizing technologies other than semiconductor based technologies, e.g. using proof masses. Proof mask based transducers have moving parts and can therefore break upon mechanical shock or magnetic overloading.
Because of this requirement, 3-axis magnetic field sensing has remained a relatively costly and complex endeavor, and the 3-axis devices may be resultingly too large to be inserted into very small spaces. In fact, common highly sensitive semiconductor based magnetic transducers are only sensitive to one single direction x-in plane of the wafer while ignoring magnetic field components in the other directions y-and z-. For many applications sensitivity in more than one direction is desired, often with space restrictions, for example in consumer goods.